There is a high incidence of infectious disease in heroin-dependent individuals. In spite of the major health issues surrounding heroin use, few studies have examined the impact of heroin on immune status. A wealth of recent data has revealed that nitric oxide plays a pivotal role in determining the outcome of exposure to certain pathogens. Our laboratory has provided the first evidence that heroin administration produces alterations of the expression of inducible nitric oxide. Specific Aim I tests the hypothesis that heroin induces alterations of nitric oxide production in models of Gram-positive, Gram-negative infection, and polymicrobial infection. The proposed studies investigate the effects of heroin on nitric oxide production in models of both Gram-negative and Gram-positive bacterial infection. These studies investigate the dose-dependent effects of heroin on the production of nitric oxide following injection of immunogenic bacterial components: lipopolysaccharide (Gram-negative), lipoteichoic acid alone (Gram-positive) and in combination with peptidoglycan. Subsequent studies will examine the effects of heroin on nitric oxide expression using live bacterial challenges with monoinfections of representative sepsis-capable strains of Gram-negative bacteroides and Gram-positive Group B streptococci, and the ceca-ligation and puncture model (CLP), a polymicrobial infection challenge. Specific Aim II tests the conditioned effects of heroin-induced alterations of nitric oxide expression in models of Gram-positive and Gram-negative infection, and polymicrobial infection. There is growing evidence that Pavlovian conditioning processes can modulate immune responses. Our laboratory has provided the first data showing that environmental stimuli paired with heroin-induce alterations of nitric oxide expression. The proposed studies extend this important discovery to models of both Gram-negative and Gram-positive bacterial infection, and the mixed infection CLP model, including assessments of acquisition and extinction of conditioned responses. Specific Aim III tests the hypothesis that the central dopaminergic systems are involved in the regulation of heroin-induced alterations of nitric oxide expression in models of infection. Our laboratory has provided new data showing that dopamine is involved in the conditioned and unconditioned effects of opioids on immune responses. The proposed investigations examine the role of central dopamine in unconditioned and conditioned heroin-induced alterations of nitric oxide expression. The proposed experiments will determine the effect of 7-OH-DPAT and the D1-selective antagonist, SCH23390, on heroin-induced alterations of nitric oxide in both the component and live models of infection. Collectively, the characterization of heroin-induced modulation of inducible nitric oxide in bacterial injections models provides a greater understanding of how opioids impact the immune system and health.